Completely self-protected overhead transformers presently use a secondary breaker to monitor secondary fault current and overload conditions in the transformer and interrupt the secondary current whenever these conditions exceed prescribed limits. A troubleshooter is then sent out to investigate the problem and, if it cannot be corrected immediately, the secondary breaker would be switched into an emergency overload mode before resetting and closing, to allow the transformer to continue operating at a higher temperature until the problem is corrected.
Switching to the emergency overload mode requires a mechanical adjustment of the latch release force for the secondary breaker, thus forcing the bimetal operator to rise to a higher temperature before the latch is released to trip out the secondary breaker.
In the transformer primary circuit breaker which is described in U.S. Pat. No. 4,435,690 issued on March 6, 1984, entitled Primary Circuit Breaker, the secondary overcurrent and/or overload conditions are monitored by a primary circuit breaker. The relatively low primary currents make a bimetal operator impractical. The temperature sensing assembly described in U.S. Pat. No. 4,435,690 includes a magnet having known holding and opening forces which are dependent on the Curie temperature of a sensing element. As the temperature of the sensing element approaches the Curie temperature, the magnet will be released to trip out the primary circuit breaker. The invention provides a means for reducing the current to the element when the transformer is switched to the emergency overload mode.